1. Field of the Invention
The invention relates to thin film strain gages.
2. Description of the Relevant Art
The accurate measurement of both static and dynamic strain, at elevated temperatures is frequently required to determine the instabilities and life-times of various structural systems, and in particular, advanced aerospace propulsion systems. Conventional strain gages are typically applied to both stationary and rotating components for this purpose but are usually limited in scope due to their intrusive nature, severe temperature limitations and difficulties in bonding.
Thin film strain sensors are particularly attractive in the gas turbine engine environment since they do not adversely effect the gas flow over the surface of a component and do not require adhesive or cements for bonding purposes. Typically, thin film strain gages are deposited directly onto the surface of a component by rf sputtering on other known thin film deposition technology and as a result are in direct communication with the surface being deformed. In general, the piezo-resistive response or gage factor (g), of a strain gage is the finite resistance change of the sensing element when subjected to a strain and can result from (a) changes in dimension of the active strain element and/or (b) changes in the resistivity (p) of the active strain element. Further, the active strain elements used in a high temperature static strain gage, must exhibit a relatively low temperature co-efficient of resistance (TCR) and drift rate (DR) so that the thermally induced apparent strain is negligible compared to the actual mechanical applied strain.
One material of choice for high temperature thin film strain gages is a wide band semiconductor, e.g. indium-tin oxide (ITO), due to its excellent electrical and chemical stability and its relatively large gage factor at high temperature. When used alone it is usually limited by relatively high TCRs as is the case for many intrinsic semiconductors. However, as disclosed herein the TCR of a self-compensated ITO strain sensor can be reduced using a metal, e.g. Pt as a thin film resistor placed in series with the active ITO strain element.
However, the proper combination of materials, patterns and dimensions to fabricate a strain gage with a predetermined TCR is mostly a matter of empirical observation, i.e. trial and error.
With the present invention, knowing the temperature range that the sensor is to be operative at and the resistivities of the materials each at a working and a reference temperature one can automatically determine the TCR of a high temperature strain gage.
Broadly the invention comprises a self-compensated strain gage sensor having an automatically determined TCR including a TCR of essentially zero. The sensor comprises a wide band semiconductor deposited on a substrate. A metal is deposited on the substrate and in electrical communication with the semi-conductor functioning as serial resistors, the length, width and thicknesses of the semi-conductor and the metal are selected based on their resistivities at selected working and reference temperatures and the TCR automatically determined.
The semiconductors can be selected from the group consisting of silicon carbide, aluminum nitride, zinc oxide, gallium nitride, indium nitride, scandium nitride, titanium nitride, chromium nitride, zirconium nitirde, boron carbide, diamond, titanium carbide, tantalum carbide, zirconium carbide, gallium phosphide, aluminum gallium nitride, zinc oxide doped with alumina, cadmium telleride, cadmium selenide, cadmium sulfide, mercury cadium telleride, zinc selenide, zinc telleride, magnesium telleride, tin oxide, indium oxide, manganates-manganese oxides with iron oxides, iron oxide-zinc-chromium oxide, iron oxide-magnesium-chromium oxide, ruthenium oxide, lithium doped nickel oxide, tantalum nitride, indium-tin oxide-gallium oxide-tin oxide and combinations thereof.
The metal resistors can be selected from the group consisting of platinum, rhodium, palladium, gold, chromium, rhenium, irridium, tungsten, molybdenum, nickel, cobalt, aluminum, copper, tantalum, alloys of platinum and rhodium and combinations thereof.
A particularly preferred semi-conductor is indium tin oxide and a particularly preferred metal is platinum.